Lead-acid batteries are an inexpensive, reliable, and rechargable storage medium for electric power. Their ability to provide a short burst of high current has made them the battery of choice to crank an automobile's starter motor for more than a century. Despite a long history of development and commercialization, and many well-studied shortcomings, the lead-acid battery is still being improved.
One area of continuing research is the materials used inside the battery. Most of them are exposed to the highly corrosive sulfuric acid that is used as the electrolyte in almost all lead-acid batteries. The materials used in the battery's positive and negative electrode assemblies, as well as the battery's separator, are expected to maintain their structural integrity for years in this corrosive environment. They should also be inexpensive and lightweight to keep the battery's costs down and energy-to-weight ratio up.
Glass fibers have become a popular choice for the separator and reinforcing mats found in lead-acid batteries. The glass fibers are relatively inert to the concentrated sulfuric acid used in the batteries, electrically insulating, and when properly arranged in a glass-fiber mat are sufficiently porous to allow the efficient migration of sulfate ions between the positive and negative electrodes. However, the glass fibers do not naturally stick together, and require a binder to hold them together in the mat. That binder needs to be resistant to the acidic environment just like the fibers themselves.
The binder compositions for glass fibers are typically organic compounds that form a polymer matrix around the fibers when cured. There are classes of organic compounds that form acid-resistant polymers, but a hydrophobic character often complements their acid resistance. Acid resistant polymers generally do not have a strong bonding affinity for the polar-protic constituents of a concentrated sulfuric acid solution, which keeps the polymers from dissolving in the acid. However, the hydrophobic character of the polymers also reduce the wettability and wickability of the mat in the battery acid, which can have several adverse consequences on battery performance. For example a battery separator with reduced wettability and wickability hinders the migration of sulfate ions between electrodes, which reduces the charging and discharging efficiency of the battery. This disadvantage is acutely felt in lead-acid batteries used in gas-electric hybrid vehicles that shut off a gasoline-powered internal combustion engine much more frequently than a conventional, gasoline-only vehicle.
There is a need for improved glass fiber mats in lead-acid batteries that resolve the tradeoff between binders with good acid resistance and binders that impart good wettability and wickability characteristics to the mat. These and other issues are addressed in the present application.